Hitherto, for constructing roofs or the eaves thereof, etc, roofing materials comprising a metal material such as a zigzag folded sheet affixed to a steel frame-based framework or roofing materials formed using concrete have been common, but where the chief structural material is a metal there is the problem of deterioration due to rusting. Furthermore, since such roofing materials are composed of metal or concrete, they are heavy and, as well as this resulting in installation difficulties at the time of assembly, there are also, for example, disadvantages from the point of view of the earthquake resistance of the building when an earthquake occurs. With metal, concrete or the like, if the weight is reduced, it then becomes more difficult to ensure satisfactory strength and rigidity.
In particular, in the case of buildings of large area requiring a construction without pillars, for example gymnasia, large halls and the like, it is necessary to form a large span without the need for supporting pillars. Thus, it is necessary for the roofing material itself to have adequate strength and rigidity. Hitherto, in such applications, precast concrete formed by reinforcing concrete with steel reinforcements has been employed. A method using, for example, approximately 24 m long precast concrete units as the roofing material is well known for gymnasia roofs. However, in the case of precast concrete, the weight per unit area is very great, at approximately 250 kg/m2, so as well as installation being difficult there is also the problem that the effects on the base structure at the time of an earthquake are considerable, making it necessary for the building to have an unyielding base structure.
Furthermore, in the case of a roofing material construction where there has been affixed a metal material comprising a flat sheet or zigzag folded sheet, shaping into a curved surface or into a three-dimensional form is difficult, so there are considerable design limitations and there are restrictions in terms of improving the design by shaping the roofing material into a desired form.
Now, while FRP materials have been used hitherto for ships and other such large structures, practically no investigations have been conducted into their application as building materials with the exception of studies into their use merely as interior decorative materials or the like. This is because FRP constructions in the shape of large structures have not been investigated in forms suitable as building materials and because associated factors relating to building materials, for example the fire-resistance and joint structures, etc, have not been fully investigated.
Furthermore, with regard to the method of forming a large structural material from an FRP, while there is the conventionally-known hand lay-up method, since the resin impregnation of the reinforcing fibre is carried out by hand in this method, the proportion of resin to reinforcing fibre in the FRP is very high and it is not possible to utilize the physical properties of the reinforcing fibre efficiently. Moreover, because the resin is handled in the open, there are considerable problems in terms of the environment.
As a means for overcoming the above there is, for example, the RTM method where the resin is injected into a moulding tool in which reinforcing fibre has already been set, but there are problems in that it is necessary to apply pressure to the moulding tool and a very expensive moulding tool is required. Furthermore, as an improvement to the RTM method, in U.S. Pat. No. 5,721,034, for example, there is proposed a method where, by cutting channels of different cross-sectional area in the core material from which the sandwich panel is composed and by injecting resin into the core material channels under vacuum, an FRP of large area is efficiently formed. However, because injection is carried out under vacuum in this method, it is necessary for the resin used to be of low viscosity. In the case of the phenolic resins which are particularly suitable for use in building material applications, it is difficult to obtain large integrally-moulded items using the above method.
Now, in order to join FRP members one to another, there is normally used the so-called lap joint technique where sheets of the same kind of material or metal sheets are arranged on the upper and lower surfaces of the FRP members and these then fastened by means of bolts or the like which pass through the FRP members. Furthermore, in another method, the FRP members are joined to one another using an adhesive. Again, there is also employed a method in which a metal frame is used as a base, with the FRP members lined-up on top thereof.
However, when joining FRP members to one another, in the case of the method of fastening using sheets arranged on the upper and lower surfaces as described above, it is necessary to provide holes in the FRP members passing through in their thickness direction, so problems arise in that the inherent strength of the FRP members is lowered and there is a risk of leaks occurring when used as a roofing material.
As examples of the above problem, there are cases where the bolting is carried out by means of through-holes or embedded metallic sleeves provided in the FRP member itself, as disclosed in JP-A-5-69487 and JP-A-9-32914. In structural terms, the joint strength in these examples depends primarily on the bearing strength of the FRP member and the bolt shearing strength. Generally speaking, an FRP is an anisotropic material based on the fibre orientation and, while its tensile strength is very high in the direction of fibre orientation, its compressive and shearing strengths are low. Thus, in the aforesaid joining method, the joint strength is governed by the compressive and shearing strengths of the FRP, and so large loads cannot be sustained.
As a method for resolving this problem, there is disclosed in JP-A-8-333807 a method where an FRP member is inserted in an opening and, furthermore, there is provided a metallic sleeve in which the FRP member is inserted and, after insertion of the member, a shaft is passed through the metallic sleeve. By this method, there is formed a structure where the shearing force and bending moment generated in the region of the joint are mitigated. However, when bending and tensile loads are applied, in addition to a compressive force acting on the FRP layer at the opening, sometimes a shearing force is produced in the embedded metal sleeve portion and a bearing pressure is produced on the FRP member, with the result that stress concentrations in these regions govern the joint strength.
On the other hand, in the case of joining using an adhesive, because the joining operation to assemble an FRP roof structure for example is generally an outdoor operation, the joining is very difficult to carry out reliably under such circumstances.
Furthermore, when carrying out re-roofing due to the deterioration of an existing roof such as that of a gymnasium, in order to protect the building itself from weather conditions of various kinds, there must be provided a temporary roof, and the provision of a temporary roof during prolonged work is both a technical problem and a problem in terms of cost.
As stated above, with regard to roofing material applications, in particular roofing materials for a large structure, hitherto there has been no use at all of FRPs.
Thus, the problems addressed by the present invention are the structure of an FRP roofing material to provide an FRP roofing member to replace conventional metallic and concrete roofing members; a method for the production thereof; the joint structure for such FRP roofing material; and the joining method.